Format size reduction of silver halide light-sensitive materials has recently advanced so much that photographic light-sensitive materials with more sensitivity and better image quality have been eagerly desired.
This desire leads to an even greater demand for photographic silver halide emulsions with still higher-level photographic properties, such as, much higher sensitivity, much higher contrast, and much better graininess and sharpness.
In order to comply with this demand, processes for producing and techniques of using tabular grains to improve sensitivity by improving the color sensitizing efficiency by sensitizing dyes, improving the relationship between sensitivity and graininess, and improving sharpness and covering power are disclosed in U.S. Pat. Nos. 4,386,156, 4,504,570, 4,478,929, 4,414,304, 4,411,986, 4,400,463, 4,414,306, 4,439,520, 4,433,048, 4,434,226, 4,413,053, 4,459,353, 4,490,458 and 4,399,215.
Techniques for improving the sensitivity of tabular grains with a specific shape are disclosed in U.S. Pat. Nos. 4,435,501 and 4,459,353 while disclose tabular grains formed by epitaxially depositing silver chloride guest grains as projections onto host tabular grains at selected surface sites.
On the other hand, epitaxial deposition of silver chloride guest grains onto host grains is disclosed in Berry and Skillman, "Surface Structures and Epitaxial Growths on AgBr Microcrystals", Journal of Applied Physics, Vol. 35, No. 7, July 1964, PP. 2165-2969.
U.S. Pat. Nos. 3,804,629 discloses that the stability of a silver halide emulsion to metal dust is improved by depositing silver chloride onto silver halide grains prior to chemical ripening of the silver halide emulsion after physical ripening and desalting of the emulsion. In this deposition, the silver chloride forms small projections on the silver halide host grains.
U.K. Patent 2,038,792A discloses a technique for selectively depositing silver chloride onto the corners of silver bromide tetradecahedral grains.
U.S. Pat. Nos. 3,505,068, 4,094,684 and 4,142,900 disclose a technique for epitaxially depositing silver chloride onto silver iodide host grains.
However, grains where silver chloride is epitaxially deposited onto host grains, or grains with projections on their surfaces, are thermodynamically so unstable that the grain shape changes when stored at an elevated temperature or when stored for a long period of time and inevitably a deterioration of sensitivity and an increase of fog occur, thus being unfavorable as silver halide emulsion-production methods.
In addition, with light-sensitive layers of a multi-layer structure having two or more emulsion layers, one emulsion layer is influenced by the other emulsion layer or layers. For example, one emulsion layer is influenced by diffusion of halide ion or the like from other emulsion layers upon coating of emulsion layers. As a result, grains with projections on their surfaces easily undergo a change in grain shape. Thus, it is difficult to provide the performance obtained by coating a single layer. Emulsions of grains with projections also have problems as to storage stability due to emulsion layer-to-emulsion layer migration of dyes, antifoggants, etc. depending upon the storage conditions such as storage temperature, storage humidity and storage period.
Further, silver halide grains with silver chloride epitaxially deposited thereon to often suffer a deterioration in graininess although sensitivity is improved. That is, this sensitizing technique is not necessarily a sufficient technique from the standpoint of the sensitivity/graininess of silver halide grains.
On the other hand, AgCl-shell grains are described in Berichte der Bunzen Gesellschaft fur Physikalische Chemie, 67, 356 (1963), U.K. Patent 1,027,146, etc.
However, the grains described in Berichte der Bunzen Gesellschaft fur Physikalische Chemie, 67, 356 (1963) are grains of cubic AgBr cores with a shell of 100-.ANG. AgCl, and the grains disclosed in the Examples of U.K. Patent 1,027,146 are grains with a thick AgCl shell which are not intended to be processed with an ordinary developer. Grains covered by the AgCl shell in a thickness as thick as described above have deteriorated graininess and concurrently dye adsorption is deteriorated.
JP-A-1-121848 and JP-A-1-26839 etc. (the term "JP-A" as used herein means an "unexamined published Japanese patent application") describe silver halide photographic emulsions containing silver bromide series grains which have no projections on the surface thereof and wherein silver chloride content of the surface silver halide layer is higher than that of the portion inside the surface.
However, the silver halide grains described therein, which structurally comprise silver halide layer-forming base grains on the inside with a silver chloride layer forming a silver halide layer on the grain surface, are not particularly limited as to the technique of forming the base silver halide layer inside the grains.
In general, silver halide grains are formed by reacting an aqueous solution of a silver salt with an aqueous solution of a halide in an aqueous colloidal solution in a reaction vessel. The single jet process which comprises adding an aqueous solution of a silver salt to a mixture of a protective colloid such as gelatin and an aqueous solution of a halide in a reaction vessel with stirring over a certain period of time and the double jet process which comprises adding an aqueous solution of a halide and an aqueous solution of a silver salt to an aqueous solution of gelatin in a reaction vessel for certain periods of time, respectively. By comparison, the double jet process provides silver halide grains having a narrower grain size distribution than the single jet process. In the double jet process, the halide composition ca be freely altered as the growth of the grains progresses.
It is known that the growth rate of silver halide grains largely depends on the concentration of silver ions (halogen ions) in the reaction solution, the concentration of the silver halide solvent, the distance between the grains, the grain size, etc. In particular, the lack of uniformity in the concentration of silver ions or halogen ions produced by the addition of an aqueous solution of a silver salt and an aqueous solution of a halide results in different growth rates, giving a non-uniformity in the resulting silver halide emulsion. In order to eliminate this problem, it is necessary to rapidly and uniformly mix and react the aqueous solution of the silver salt with the aqueous solution of the halide in the aqueous solution of the colloid so as to provide uniformity in the concentration of the silver ions or the halogen ions in the reaction vessel. In the conventional process which comprises adding an aqueous solution of a halide and an aqueous solution of a silver salt to the surface of an aqueous solution of a colloid in a reaction vessel, portions of higher halogen ion and silver ion concentrations are produced in the vicinity of the location at which each reaction solution is added. This results in difficulty in the preparation of uniform silver halide grains. Methods for eliminating such an uneven concentration distribution are disclosed in U.S. Pat. No. 3,415,650, and 3,692,283,and U.K. Patent 1,323,464. In these methods, a reaction vessel is filled with an aqueous solution of a colloid. The reaction vessel is equipped with a rotary convex cylindrical hollow mixer having slits in the wall thereof (filled with an aqueous solution of a colloid, preferably composed of an upper chamber and a lower chamber partitioned by a disc in the vessel). The axis of rotation of the mixer is vertical. The aqueous solution of the halide and the aqueous solution of the silver salt are supplied into the mixer, which is rotating at a high speed, at the top and bottom open ends through feed pipes so that they are rapidly mixed and reacted with each other. (If there are two chambers in the mixer, the two aqueous solutions supplied into the respective chamber are first diluted with an aqueous solution of the colloid present therein, and then they are rapidly mixed and reacted with each other in the vicinity of the outlet slits.) The silver halide grains thus formed are then introduced into the aqueous solution of the colloid in the reaction vessel by the centrifugal force produced by the rotation of the mixer.
On the other hand, U.S. Pat. No. 4,289,733 discloses a method for eliminating an uneven concentration distribution to prevent non-uniform growth of grains. In this method, an aqueous solution of a halide and an aqueous solution of a silver salt are separately supplied into a mixer filled with an aqueous solution of the colloid in a reaction vessel filled with an aqueous solution of the colloid from the bottom open end of the mixer through feed pipes. These reaction solutions are rapidly agitated and mixed with each other by a lower agitator (turbine impeller) provided in the mixer to effect the growth of silver halide. The resulting silver halide grains are immediately introduced into the aqueous solution of the colloid in the reaction vessel from the upper open end of the mixer by an upper agitator provided above the lower agitator.
JP-A-57-92523 discloses a preparation method which is intended to eliminate such a non-uniformity in concentration. In this method, an aqueous solution of a halide and an aqueous, solution of a silver salt are separately supplied into a mixer filled with an aqueous solution of a colloid in a reaction vessel filled with an aqueous solution of the colloid from a lower open end of the mixer. The two reaction solutions are diluted with the aqueous solution of the colloid and then rapidly mixed with each other by a lower agitator provided in the mixer. The resulting silver halide grains are immediately introduced into the aqueous solution of the colloid in the reaction vessel from an upper open end of the mixer. In this method and apparatus therefor, the two reaction solutions which have been diluted with the aqueous solution of the colloid are passed through the clearance between the inner wall of the mixer and the tip of the agitator without being passed through the gap between the impellers so that they are rapidly mixed and reacted with each other under a shearing force in the clearance to form silver halide grains.
These methods and apparatus can thoroughly eliminate the uneven distribution of concentration of silver ions and halogen ion in the reaction vessel. However, an uneven concentration distribution still exists in the mixer. In particular, a relatively large uneven concentration distribution exists in the vicinity of the nozzle through which the aqueous solution of the silver salt and the aqueous solution of the halide are supplied of the portion under the agitator and of the portions agitated. Furthermore, the silver halide grains supplied into the mixer together with the protective colloid are passed through these portions having an uneven concentration distribution. It should be particularly noted that the silver halide grains rapidly grow in these portions. In other words, these preparation methods and apparatus therefor are disadvantageous in that an uneven concentration distribution exists in the mixer, and the growth of grains takes place rapidly in the mixer, failing to accomplish the object of allowing uniform growth of the silver halide under conditions free of a concentration distribution difference.
In order to accomplish a more efficient mixing so as to eliminate the uneven concentration distribution of silver ions and halogen ions, additional attempts have been made. For example, a reaction vessel and a mixer are independently provided. An aqueous solution of a silver salt and an aqueous solution of a halide are supplied into the mixer where they are rapidly mixed with each other to effect the growth of silver halide grains. In a preparation method and apparatus disclosed in U.K. Patents 1,591,608 and 1,243,356 an aqueous solution of a protective colloid (containing silver halide grains) is pumped from the bottom of a reaction vessel and circulated therein. A mixer is provided in the course of the circulation system. An aqueous solution of a silver salt and an aqueous solution of a halogen are supplied into the mixer where they are rapidly mixed with each other to effect the growth of silver halide grains. In a method disclosed in U.S. Pat. No. 3,897,935, an aqueous solution of a protective colloid (containing silver halide grains) is pumped from the bottom of a reaction vessel and circulated therein. An aqueous solution of a halide and an aqueous solution of a silver salt are pumped into the course of the circulation system. In a preparation method and apparatus disclosed in JP-A-53-47397, an aqueous solution of a protective colloid (containing silver halide grains) is pumped from the bottom of a reaction vessel and circulated therein. An aqueous solution of an alkali metal halide is first introduced into the circulation system. The aqueous solution of an alkali metal halide is diffused into the system until the system becomes uniform. Thereafter, an aqueous solution of a silver salt is introduced into and mixed with the system to form silver halide grains. These methods enable independent altering of the rate at which the aqueous solutions flow from the reaction vessel to the circulation system and the agitation efficiency of the mixer, making it possible to effect growth of grains under a condition of a more uniform concentration distribution. However, these methods are still disadvantageous in that the crystalline silver halide which has been delivered from the reaction vessel together with the protective colloid is subject to rapid growth at the inlet portion from which the aqueous solution of the silver salt and the aqueous solution of the halide are introduced into the system. Therefore, in these methods, it is impossible, in principle, to eliminate such a concentration distribution difference in the mixing portion or in the vicinity of the inlet portion. That is, the object of allowing uniform growth of silver halide under a condition free of concentration distribution cannot be accomplished.
As has been described hereinabove, in forming the silver halide layer to be used as a base layer as described in JP-A-1-121848 and JP-A-1-26839, it is impossible to grow the base silver halide layer uniformly in the absence of a concentration gradation of silver and halide ions by employing conventionally known processes for forming silver halide grains.